Three of the smaller things I think would be nice,
First: A different texture between disturbed soil and the clay texture, so I can actually see what I'm digging on at any given time.
Second: Backfill with gravel. Or something to that effect. would be nice to be able to take rock and backfill areas with it for flattening.
Third: a way to fill in on top of clay pits, most likely destroying the clay pit itself, but would still be nice, as it makes no sense to not be able to refill a clay area.